Hovercraft are generally fitted with a compressor to generate the necessary air-cushion within a rubber skirt and at least one drive engine for taking straight and/or curve courses. The rubber skirt consists of a tubular, peripheral top skirt, a so-called back skirt, and a flexible, finger-type surround connected to it, a so-called finger skirt. Because these terms are common in hovercraft engineering, they will also be used in the following description for the sake of simplicity. The finger skirt can move laterally in elastic fashion and serves as a sealing collar against the ground or the surface of the water. When a curve course is taken, the hovercraft is usually steered by laterally deflecting the jet of air, for example, with a vertical rudder, such as is familiar from aircraft construction.
In such hovercraft, the bodywork tilts about the longitudinal axis of stability, the transverse axis of stability or both axes simultaneously, if the center of gravity shifts due to varying or non-uniform loading with passengers or cargo, for example.
In the worst case, this inclination about one of the axes of stability, caused by a shift in the center of gravity, can cause the rubber skirt of the hovercraft to dip into the water while travelling. In this case, the rubber skirt would be compressed to a more or less pronounced degree and the buoyancy of the air-cushion reduced in this area. The consequence of this is that the hovercraft dips even further into the water and could even flip over.
Another effect which occurs following a shift in the center of gravity is thoroughly comparable to the effect of side-winds. This effect arises as a result of the varying gap between the rubber skirt and the ground or the surface of the water and the resultant varying release of the air from the air-cushion. The force component arising in this context pushes the hovercraft in the direction of the shift in the center of gravity.
In principle, measures to compensate for the shift in the center of gravity in hovercraft are possible by using additional trimming weights, for example. However, these measures lead to an additional weight load and, thus, to increased energy consumption.
Another possibility for trimming is familiar from FR-A-2 187 586, in which the finger skirts are each connected to the bodywork with cables. These cables define the maximum distance between the finger skirts and the bodywork. The connection points of the finger skirts to the cables are interconnected on each side by a longitudinal cable, so that the maximum distance of the finger skirts to the bodywork can be shortened by pulling the cable in the longitudinal direction. However, it is not possible to enlarge the distance to the bodywork, resulting in only limited possibilities for adjustment. In order to achieve the greatest possible trimming flexibility, the finger skirts of each side are split into two groups, each of which can be adjusted independently of the other by means of a longitudinal cable. This means that a total of four groups of finger skirts can be adjusted independently of one another.
Trimming a hovercraft with such an arrangement is complicated, as adjusting the finger skirts of one group is only capable of reducing the buoyancy of the air-cushion at this point, i.e. the finger skirts must be adjusted where the least load is present.